An electric power steering apparatus for applying a steering assist force to a steering mechanism of an automobile by a rotating force of a motor transmits a driving force of the motor by a transmitting mechanism such as a gear or a belt through a reduction gear to apply the steering assist force to a steering shaft or a rack shaft. An example of a brief structure of such an electric power steering apparatus is shown in FIG. 7 and will be described.
A shaft 102 of a steering wheel 101 is coupled to tie rods 106 of steered wheels through a reduction gear 103, universal joints 104a and 104b, and a pinion rack mechanism 105. The shaft 102 is provided with a torque sensor 107 for detecting steering torque of the steering wheel 101, and a motor 108 for assisting a steering force of the steering wheel 101 is coupled to the shaft 102 through the reduction gear 103.
A control of the motor 108 is very important for the electric power steering apparatus mentioned above, and it is necessary to control the motor 108 in correspondence to a steering operation, a vehicle speed or the like. For example, FIG. 8 is a control block diagram of the motor 108 using a vector control which has been well known conventionally.
Describing the control block diagram, a torque command value Tref, a rotation angle θ and an angular velocity ω are input to a current command value calculating portion 204. The torque command value Tref is calculated by a torque command value calculating portion 220 corresponding to a torque command value calculating means on the basis of a steering torque Tr detected by the torque sensor 107, and the rotation angle θ corresponds to an electrical degree of the motor 108. The current command value calculating portion 204 calculates a current command value Iqref of a q-axis component and a current command value Idref of a d-axis component. Generally, the current command value Iqref changes in proportion to the torque command value Tref, and the current command value Idref equals to 0 (generally, a relation Idref=0 is established). On the other hand, an angle detecting device for detecting the rotation angle θ of the motor 108 is installed. The angle detecting device includes an encoder, a Hall sensor or the like, however, employs an angular resolver 201 in this case. Since a signal output from the angular resolver 201 does not directly indicate the rotation angle θ, a position detecting circuit 202 for executing a calculating process is necessary, and an RDC circuit or the like is used for the angular resolver 201. If the rotation angle θ output from the position detecting circuit 202 is input to a differential calculating portion 203, the angular velocity ω is calculated. The rotation angle θ and the angular velocity ω are used in a control mentioned below.
The present control block diagram employs a feedback control as an example, and it is necessary to execute a feedback control by detecting actual motor currents Ia, Ib and Ic of the motor 108, with respect to the current command values Iqref and Idref mentioned above. Specifically, the motor currents Ia, Ib and Ic are detected in the current detecting devices 205-1, 205-2 and 205-3, and are converted to motor currents Iq and Id in a three-phase/two-phase conversion portion 206 for the vector control. The rotation angle θ of the motor mentioned above is used for the conversion. Next, the motor currents Iq and Id are fed back to subtraction portions 207 and 207-2, a deviation ΔIq between the current command value Iqref and the motor current Iq is calculated by the subtraction portion 207-1, and a deviation ΔId between the current command value Idref (normally Idref=0) and the motor current Id is calculated by the subtraction portion 207-2.
In order to eliminate the deviations, they are input to a proportional integral (PI) calculating portion 208, and voltage command value Vd and Vq are output. Further, since it is necessary that the actual motor 108 supplies a three-phase current, the voltage command values Vd and Vq are converted to three-phase voltage command values Va, Vb and Vc by a two-phase/three-phase conversion portion 209. A PWM control portion 210 generates a PWM control signal on the basis of the voltage command values Va, Vb and Vc, and an inverter circuit 211 supplies the current to the motor 108 on the basis of the PWM control signal, and supplies the motor currents Ia, Ib and Ic in such a manner that the deviations with respect to the current command value Iqref and Idref disappear.
The control mentioned above corresponds to a basic control with respect to the motor 108, however, a capacity of the motor 108 has a limit. In the case of steering the steering wheel at a high speed, a power of the motor runs short. Accordingly, there is a case that a high-speed rotation is achieved by restricting an output torque of the motor, and a control having a constant power is executed. In order to achieve the control mentioned above, a control method called as a field weakening control is used. A relation q-axis current command value Idref=0 is generally established, however, in the case of the field weakening control, the relation Idref=0 is not equivalently established. The d-axis current command value Idref corresponds to a current component corresponding to a field magnetic flux, and increasing the d-axis current command value Idref in a negative direction is equivalent to weakening the field magnetic flux on the axis d. If the field magnetic flux is weakened, a counter electromotive force becomes small. Accordingly, it is possible to rotate the motor at a higher rate.
There is provided a device improving a feeling of a steering in a rapid wheel steering, by executing the field weakening control.
Specifically, in the case that the field weakening control is achieved by a advance angle control of the vector control, the following numerical expression 1 is executed with respect to the voltage command values Vd and Vq, in a two-phase/three-phase conversion portion 209, on the basis of an angle φ of the advance angle calculated by a advance angle calculating portion 212 described in detail below, in FIG. 8, and the voltage command values Va, Vb and Vc are calculated.
                    Numerical        ⁢                                  ⁢        Expression        ⁢                                  ⁢        1                                                                      [                                                    Va                                                                    Vb                                                                    Vc                                              ]                =                              [                                                                                                      -                      cos                                        ⁢                                                                                  ⁢                                          (                                              θ                        +                        ϕ                                            )                                                                                                            sin                    ⁢                                                                                  ⁢                                          (                                              θ                        +                        ϕ                                            )                                                                                                                                                              -                      cos                                        ⁢                                                                                  ⁢                                          (                                              θ                        +                        ϕ                        -                                                  2                          ⁢                                                      π                            /                            3                                                                                              )                                                                                                            sin                    ⁢                                                                                  ⁢                                          (                                              θ                        +                        ϕ                        -                                                  2                          ⁢                                                      π                            /                            3                                                                                              )                                                                                                                                                              -                      cos                                        ⁢                                                                                  ⁢                                          (                                              θ                        +                        ϕ                        +                                                  2                          ⁢                                                      π                            /                            3                                                                                              )                                                                                                            sin                    ⁢                                                                                  ⁢                                          (                                              θ                        +                        ϕ                        +                                                  2                          ⁢                                                      π                            /                            3                                                                                              )                                                                                            ]                    ⁡                      [                                                            Vd                                                                              Vq                                                      ]                                                          
In this numerical expression 1, the current command value (the d-axis current command value) for weakening the field is calculated by being advanced at the angle φ. In other words, in the advance angle control, the field current command value for weakening the field means the angle φ of the advance angle. In other words, a component generated by the angle φ generates an effect of weakening the field, in the voltage command values Va, Vb and Vc.
Next, a description will be given of a specific calculating method of the angle φ of the advance angle with reference to FIG. 9. A base angular velocity ωb is calculated in a conversion portion 212a by setting the torque command value Tref to an input, while a mechanical angular velocity ωm is calculated in a mechanical angle calculating portion 212b having an angular velocity ω of the motor 108 calculated in the differential calculating portion 203, and the angle φ is calculated in an arcCOS calculating portion 212c, on the basis of a relation angle φ=arcCOS (ωm/ωb). The angle φ is a value which first appears at a time when the mechanical angular velocity ωm of the motor becomes higher than the base angular velocity ωb, as is known from an expression angle φ=arcCOS (ωm/ωb), in other words, the field weakening control is executed at a time when the mechanical angular velocity ωm of the motor becomes higher than the base angular velocity ωb.
The control mentioned above is the basic of the field weakening control, however, in order to achieve a electric power steering apparatus having a good wheel steering feeling all the time in various steering wheel operation and vehicle speed conditions, by applying various improvements to the field weakening control, the field weakening control is executed so as to correspond to a change of the steering operation, the vehicle speed or the like.
For example, in Japanese Patent Application Laid-Open (JP-A) No. 2003-40128, an improvement of adjusting the current command value Idref in correspondence to the vehicle speed is applied to the d-axis current command value Idref. The improvement content employs an improving method of making the q-axis current command value Iqref which changes in proportion to an output torque large by making smaller than the current command value Idref in the case of considering no vehicle speed, in the case that the vehicle speed is high, in order to secure the output torque of the motor preferably. In other words, when the vehicle speed is low, the wheel steering feeling is improved by making the current command value Idref smaller in accordance that the vehicle speed becomes higher, while keeping the current command value Idref normal, thereby securing the output torque preferably.
As mentioned above, it is very important in view of a performance improvement for improving a wheel steering feel of the electric power steering apparatus to control the d-axis current control value Idref. On the other hand, there is a problem that the greater the d-axis current Id affecting the field current is, the larger the noise generated by the motor is. In this case, the noise in the vehicle is caused by a noise generated between a tire and a road surface, a wind noise of a vehicle body and the like, in addition to the noise generated by the motor of the electric power steering apparatus. Further, the noise such as the noise generated between the tire and the road surface, the wind noise of the vehicle body and the like has a characteristic that the noise becomes larger in accordance that the vehicle speed becomes higher. In other words, when the vehicle speed is low, the motor noise generated by the current command value Idref largely affects the noise in the vehicle. Since the noise is large due to the other reasons than the motor at a time when the vehicle speed is high, the motor noise does not largely affect the noise in the vehicle.